This invention belongs generally to the field of .beta.-lactam antibiotics; more particularly, it relates to a process whereby a 3-methyl cephem, 1-carba(1-dethia)cephem, or 1-oxa(1-dethia)cephem is photochemically converted to the corresponding 3-exo-methylene derivative. Such derivatives provide a useful functionality in the 3-position for further derivatization. For example, the 3-exomethylene cepham may be ozonized to provide the 3-(keto)-enol, which, in turn, may be halogenated to form 3-halo cephems [see, for example, S. Kukolja and R. R. Chauvette in "Chemistry and Biology of .beta.-lactam Antibiotics", R. B. Morin and M. Gorman, Eds., Vol. I, Ch. 2, pp. 93-198, Academic Press (1982)].